线程问题;
看来只有在其他线程调用pthread_cond_notify之前调用pthread_cond_wait时，条件variables才起作用。 如果之前发生通知，等待会卡住;
我的问题是 什么时候应该使用条件variables？ 调度程序可以抢占线程并在等待之前通知已经发生;
等待信号量不存在这个问题 – 这些有一个计数器，还有什么时候是一个条件variables更好？
这是一个testing
**** file condvar.c #include <pthread.h> #include <stdio.h> #include <stdlib.h> // ****** // test of conditional variables; // if cond-var is notified before wait starts, then wait never wakes up !!! // better to use semaphores than this crap. // ****** pthread_mutex_t cond_var_lock = PTHREAD_MUTEX_INITIALIZER; pthread_cond_t cond_var = PTHREAD_COND_INITIALIZER; int wait_first = 1; void *tfunc(void *arg) { (void) arg; if (!wait_first) sleep(1); fprintf(stderr,"on enter cond_var_lock %lx\n", pthread_self()); pthread_mutex_lock( &cond_var_lock); fprintf(stderr,"before pthread_cond_wait %lx\n", pthread_self()); pthread_cond_wait( &cond_var, &cond_var_lock); fprintf(stderr,"after pthread_cond_wait %lx\n", pthread_self()); pthread_mutex_unlock( &cond_var_lock); fprintf(stderr,"after exit cond_var_lock %lx\n", pthread_self()); return 0; } int main(int argc, char *argv[]) { pthread_t th; if (argc > 0) wait_first = atoi( argv[1] ); if (wait_first) { fprintf(stderr,"********* Wait first ***********\n"); } else { fprintf(stderr,"********* Notify first *********\n"); } pthread_create( &th, 0, tfunc, 0 ); if (wait_first) { sleep(1); } fprintf(stderr, "! on enter cond_var_lock %lx\n", pthread_self()); pthread_mutex_lock( &cond_var_lock); fprintf(stderr, "! before pthread_cond_signal %lx\n", pthread_self()); pthread_cond_signal( &cond_var ); fprintf(stderr, "! after pthread_cond_signal %lx\n", pthread_self()); pthread_mutex_unlock( &cond_var_lock); fprintf(stderr, "! after exit cond_var_lock %lx\n", pthread_self()); sleep(5); return 0; } **** file test.sh #!/bin/sh set -e set -x gcc condvar.c -o condvar -lpthread ./condvar 1 ./condvar 0 **** test output Output: + gcc condvar.c -o condvar -lpthread + ./condvar 1 ********* Wait first *********** on enter cond_var_lock b7779b70 before pthread_cond_wait b7779b70 ! on enter cond_var_lock b777a6c0 ! before pthread_cond_signal b777a6c0 ! after pthread_cond_signal b777a6c0 ! after exit cond_var_lock b777a6c0 after pthread_cond_wait b7779b70 after exit cond_var_lock b7779b70 + ./condvar 0 ********* Notify first ********* ! on enter cond_var_lock b785c6c0 ! before pthread_cond_signal b785c6c0 ! after pthread_cond_signal b785c6c0 ! after exit cond_var_lock b785c6c0 on enter cond_var_lock b785bb70 before pthread_cond_wait b785bb70 应该使用条件变量作为等待并通知的地方。 他们不是条件本身，他们不是事件。 该条件包含在周围的编程逻辑中。 条件变量的典型使用模式是
// safely examine the condition, prevent other threads from // altering it pthread_mutex_lock (&lock); while ( SOME-CONDITION is false) pthread_cond_wait (&cond, &lock); // Do whatever you need to do when condition becomes true do_stuff(); pthread_mutex_unlock (&lock);
另一方面，表示条件变量的线程通常看起来像
// ensure we have exclusive access to whathever comprises the condition pthread_mutex_lock (&lock); ALTER-CONDITION // Wakeup at least one of the threads that are waiting on the condition (if any) pthread_cond_signal (&cond); // allow others to proceed pthread_mutex_unlock (&lock)